This invention pertains to a method and apparatus for controlling the brightness of x-ray images during fluoroscopy and during cinerecording.
The automatic brightness control (ABC) subsystem is that part of a diagnostic x-ray control system which keeps the light of the output image from an x-ray image intensifier constant during fluoroscopy to compensate for variations in attenuation due to scanning over different thicknesses and densities of the body and for variations in system geometry. The output image is viewed by a video camera and displayed on a television monitor. Usually, the x-ray system is equipped for recording images with cine and photospot cameras and on various magnetic recording media.
In the fluoroscopic mode, x-ray exposures are usually continuous and of long duration so x-ray tube current and, hence, x-ray photon output and dose rate are relatively low. Government regulations require that during fluoroscopy dose rate shall not exceed 10 Roentgens per minute (10R/min) at the plane where the x-ray beam enters the patient.
Automatic brightness control systems commonly have two control loops. One loop responds to brightness variation by automatically adjusting x-ray tube anode-to-cathode milliamperage (mA) and kilovoltage applied to the anode of the tube in an attempt to keep the brightness of the image intensifier output phosphor constant. The other loop provides for adjusting the gain of the video camera in response to brightness. One problem with two control loop systems acting independently is that when the x-ray beam attenuation varies as it scans over the body, or any opaque x-ray material in the bloodstream flows into the field of view or the x-ray tube focal spot-to-image distance (SID) changes, both controls try to correct brightness. This inevitably results in overshoot by one or the other loops so one has to oscillate back to the control level of the other. Thus, with prior art ABC systems there can be noticeable blooming, darkening, brightening and flickering of the displayed image during x-ray beam attenuation changes and imaging system geometry changes.
The matter of keeping output image brightness constant is complicated by the interaction of selected x-ray tube exposure factors. X-ray tube anode-to-cathode mA depends mainly on the cathode or filament temperature while a constant kilovoltage (kV) is applied to the x-ray tube anode. At lower kilovoltage such as around 60 kV electron emission from the cathode is limited largely by space charge about the cathode. As applied kV is increased, as is necessary to penetrate more dense or thicker areas in a patient, the space charge effect is reduced and there is a non-linear relation between kV and x-ray intensity or x-ray photon output from the tube. Image brightness is directly proportional to x-ray tube mA and not directly proportional to applied kV. Brightness also varies as the square of the distance between the x-ray tube focal spot and the image plane.
Within an applied kV allowable range of 60 kV to 120 kV, x-ray tube mA is the preferred parameter to regulate for maintaining constant intensifier output image brightness during fluoroscopy since brightness is directly proportional to mA. X-ray tube photon output is directly proportional to mA. Image contrast increases with increasing tube mA and concurrent output photon intensity so that maximum information is obtainable from suitably high contrast images. There are, however, limits on the level of mA affected by the fact that the patient entrance dose rate must always be kept at or below 10 R/min. For fluoroscoping thick highly attenuating regions of the body, kV must be increased and mA decreased to maintain 10 R/min. Unfortunately, as kV and penetrating power of the x-ray photons increases, contrast in the image decreases. Thus, small differences in tissue density are less likely to be perceived.
Generally, prior practice has been to detect the need for more brightness and use a servo loop to automatically increase mA up to its available limit in an attempt to achieve the necessary brightness increase. In some system designs, kV is increased manually or automatically if the desired brightness level is not achievable with current control. If proper brightness cannot be obtained within the tube current, kV, SID and 10 R/min. constraints, the gain of the video camera is increased. Increasing video camera gain is the least desirable way of increasing brightness because noise is amplified as much as picture information, but no gain in information results because it is limited already by the limit on x-ray photon input to the patient's body. Prior art brightness control systems, adjusting mA, kV and video gain are essentially step functions which result in a lack of smooth unnoticeable transition from one function to another and, hence, cause noticeable flickering and increasing and decreasing brightness as x-ray attenuation and system geometry change during fluoroscopy.